1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Description of the Prior Art
In an electric field, the molecules of liquid crystalline nematic and cholesteric compounds or mixtures which possess a negative anisotropy of the dielectric constants (i.e. .epsilon..sub..parallel. &lt;.epsilon..sub..perp.) are oriented with their longitudinal axes perpendicular to the field direction. .epsilon..sub..parallel. signifies the dielectric constant along the longitudinal axis of the molecule and .epsilon..sub..perp. signifies the dielectric constant perpendicular thereto.
This dielectric field effect is used for the control of the optical transmissivity in various liquid crystal indicators. For example, the effect is utilized in liquid crystal cells of the light scattering type (dynamic scattering), of the so-called DAP type or of the guest-host type [guest-host interaction; Applied Physics Letters 13(1968) 91].
These "guest-host cells" comprise essentially a condenser-like structure with at least one electrode plate being transparent and a dielectric being formed from nematic or cholesteric liquid crystal materials which contain one or more dichroic dyes. In the cells, the longitudinal axes of these colouring substances and the liquid crystals align parallel to each other. Since the colouring substances usually have positive dichroism, their transition moment of the absorption of visible light lies approximately in the direction of the longitudinal molecular axis of the dye. That is, one sees a colour when viewing in a direction perpendicular to the longitudinal axis of the dyes. Consequently, the orientation of the liquid crystal and dyes with their molecular axes parallel to the surface of the plates generally corresponds to the coloured state. Also, the homeotropic orientation (longitudinal molecular axes perpendicular to the surface of the plates) generally corresponds to the colourless condition of the cell.
When a liquid crystal with positive dielectric anisotropy is used (.epsilon..sub..parallel. &gt;.epsilon..sub..perp.), its homogeneous orientation (i.e. longitudinal axis of molecules is parallel to surface of electrode, which is achieved by suitably treating the surface of the electrode plates) becomes homeotropic (i.e. longitudinal axis of molecules is perpendicular to surface of the electrodes) by the application of a voltage. Consequently, from the field-off to the field-on state, the cell is switched from "coloured" to "colourless". In this manner, colourless symbols are shown on a coloured background. With a liquid crystal having negative dielectric anisotropy (.epsilon..sub..parallel. &lt;.epsilon..sub..perp.), its homeotropic orientation (i.e. by suitably treating the surface of the electrode plates) is arranged parallel to the electrode surfaces by the application of a voltage. Thus, with the field-on state there results a coloured image elements on a colourless background.
Further, for the improvement of the multiplex ratio in the multiplex control of liquid crystal indicators (especially of rotation cells and guest-host cells), there has been proposed a two-frequency matrix addressing procedure (e.g. German Offenlegungsschriften No. 28 56 134 (Great Britain Pat. No. 2,013,014) and No. 29 07 940 (Great Britain Pat. No. 2,020,075)). This procedure makes use of the fact that the dielectric anisotropy of nematic liquid crystals having a positive anisotropy of the dielectric constants upon application of a low-frequency voltage, is negative in the case of high frequencies. To maintain a relatively low energy consumption, the "cross-over frequency" f.sub.c (dielectric relaxation frequency at which .epsilon..sub..parallel. =.epsilon..sub..perp.) of such liquid crystals should be at most 20 kHz or smaller. Further, the absolute dielectric anisotropies should be as large as possible not only below but also above the cross-over frequency. Disadvantageously, at frequencies above the cross-over frequency, the substances which are especially suitable for the two-frequency procedure generally have a smaller absolute dielectric anisotropy than below the cross-over frequency. This disadvantage, however, can be eliminated by adding one or more compounds with negative dielectric anisotropy and establishing suitable relaxation behavior.
Furthermore, liquid crystals which in the case of high frequencies have a negative dielectric anisotropy can, however, also be controlled by switching-on and switching-off an alternating current of high frequency. The liquid crystals thereby behave as customary liquid crystals with negative anisotropy of the dielectric constants.
A series of liquid crystalline compounds with weakly negative dielectric anisotropy has already hitherto been synthesized. However, still relatively few liquid crystals with large negative anisotropy of the dielectric constants are known. Moreover, the latter generally have disadvantages such as poor stability in mixtures, high melting points, high viscosity, strong smectic tendencies and chemical instability. There accordingly exists a great need for further improved compounds with negative anisotropy of the dielectric constants which can be utilized in a wide variety of display applications.